May I ask why? Does it help on the backend with spammers somehow, or maybe with the problems we've been having with the site crashing?

Of course, its really none of my business... I was just wondering.

vBulletin (the BB software this forum uses) by default has always had a cookie timeout of 15 minutes. If this was disabled, people could log into the forum once and then would stay logged into it forever if they didn't specifically log out when they leave, which many don't. It's more a security setting than anything.